The recent progress of a degassing technology in the steel making industry has made it possible to manufacture ultra-low-carbon steel in which a carbon (C) content is reduced to up to 30 ppm at a relatively low cost in a large quantity. Steel known as IF steel (abbreviation of interstitial atoms free steel) comprising the above-mentioned ultra-low-carbon steel added with at least one of niobium (Nb), titanium (Ti), boron (B) and zirconium (Zr), is popularly used as a preferred material for manufacturing, through a continuous annealing treatment, a cold-rolled steel sheet for ultra-deep drawing of the EDDQ (abbreviation of excellent deep drawing quality)-class required to have deep drawability and non-aging property.
IF steel commonly used as a material for a continuously annealed cold-rolled steel sheet is ultra-low-carbon steel added with any one or both of titanium and niobium. Titanium is a strong element forming carbide and nitride in steel, and furthermore, titanium has a function of fixing sulfur in steel by forming sulfide through combination with sulfur in steel. IF steel added with titanium (hereinafter referred to as "Ti-IF steel") therefore provides an advantage of permitting stable availability of very excellent deep drawability and ductility within a wide range of a chemical composition of steel.
However, since titanium is an element easily oxidized, titanium oxide produced in molten Ti-IF steel during the continuous casting thereof, adheres and accumulates onto the surface of a bore of a pouring nozzle of a tundish, thus causing reduction or clogging of the bore, or surface defects of a slab are caused by titanium oxide. Addition of titanium in an amount sufficient to completely fix carbon in steel in the form of titanium carbide (TiC) to steel, causes a degradation in grain boundary strength of the annealed cold-rolled steel sheet, and upon subjecting the annealed cold-rolled steel sheet to the deep drawing, a problem of secondary-work embrittlement is caused in the annealed cold-rolled steel sheet. For the solution of secondary-work embrittlement, addition of boron in a slight amount to steel is known to be effective. Addition of boron to steel however results in deterioration of deep drawability of the cold-rolled steel sheet.
There is known another IF steel added with niobium (hereinafter referred to as "Nb-IF steel") as steel solving the above-mentioned problems. In Nb-IF steel, in which carbon in steel is fixed in steel in the form of niobium carbide (NbC), a cold-rolled steel sheet excellent in deep drawability is available, as in Ti-IF steel. A problem in Nb-IF steel is however that a range of an appropriate niobium content is tight. Since surface defects of a slab are hardly caused by oxide inclusions in Nb-IF steel, on the other hand, it is not necessary to scarf the surface of a continuously cast Nb-IF steel slab. This provides an advantage that it is possible to manufacture a hot-rolled steel strip from a high-temperature continuously cast slab of Nb-IF steel by the application of a method known as the hot-direct rolling method comprising directly hot-rolling a slab without reheating same. When using IF steel as a material for an alloying-treated zinc dip-plated cold-rolled steel sheet, it is known that adhesiveness of an alloying-treated zinc dip-plating layer to a cold-rolled steel sheet is improved more in Nb-IF steel or in IF steel added with both niobium and titanium (hereinafter referred to as "Nb-Ti-IF steel") than in Ti-IF steel.
With a view to further improving properties of the above-mentioned Ti-IF steel or Nb-IF steel, various methods have been proposed as described below.
(1) As a method for manufacturing a cold-rolled steel sheet having desired properties, which uses Nb-Ti-IF steel as a material, Japanese Patent Publication No. 61-32,375 published on Jul. 26, 1986 discloses a method for manufacturing an ultra-deep drawing cold-rolled steel sheet, which comprises the steps of:
hot-rolling a steel slab consisting essentially of:
carbon (C): up to 0.007 wt. %, PA1 silicon (Si): up to 0.8 wt. %, PA1 manganese (Mn): up to 1.0 wt. %, PA1 phosphorus (P): up to 0.1 wt. %, PA1 aluminum (Al): from 0.01 to 0.1 wt. %, PA1 nitrogen (N): up to 80 ppm, PA1 titanium (Ti): from 0.010 to 0.037 wt. %, PA1 niobium (Nb): from 0.003 to under 0.025 wt. %, PA1 where, PA1 the balance being iron (Fe) and incidental impurities; then PA1 carbon (C): up to 0.004 wt. %, PA1 silicon (Si): up to 0.1 wt. %, PA1 manganese (Mn): up to 0.5 wt. %, PA1 phosphorus (P) : up to 0.025 wt. %, PA1 sulfur (S): up to 0.025 wt. %, PA1 nitrogen (N): up to 0.004 wt. %, PA1 aluminum (Al): from 0.01 to 0.10 wt. %, PA1 titanium (Ti): from 0.01 to 0.04 wt. %, PA1 niobium (Nb): from 0.001 to 0.010 wt. %, PA1 boron (B): from 0.0001 to 0.010 wt. %, PA1 where PA1 and PA1 the balance being iron (Fe) and incidental impurities, PA1 carbon (C): from 0.0010 to 0.010 wt. %, PA1 silicon (Si): up to 0.5 wt. %, PA1 manganese (Mn): up to 1.4 wt. %, PA1 phosphorus (P): up to 0.05 wt. %, PA1 sulfur (S): up to 0.020 wt. %, PA1 acid-soluble aluminum (sol.Al): from 0.005 to 0.10 wt. %, PA1 nitrogen (N): up to 0.0040 wt. %, PA1 titanium (Ti): up to 0.08 wt. %, PA1 where, Ti/(C+N).gtoreq.3.0, PA1 boron (B): up to 0.0006 wt. %, PA1 and PA1 the balance being iron (Fe) and incidental impurities, PA1 carbon (C): up to 0.003 wt. %, PA1 silicon (Si): up to 0.1 wt. %, PA1 manganese (Mn): from 0.05 to 1.0 wt. %, PA1 phosphorus (P): from 0.005 to 0.1 wt. %, PA1 sulfur (S): up to 0.02 wt. %, PA1 aluminum (Al): from 0.02 to 0.1 wt. %, PA1 nitrogen (N): up to 0.0030 wt. %, PA1 titanium (Ti): from 0.03 to 0.1 wt. %, PA1 boron (B): from 0.0003 to 0.0010 wt. %, PA1 and PA1 the balance being iron (Fe) and incidental impurities, PA1 carbon (C): up to 0.004 wt. %, PA1 silicon (Si): up to 0.1 wt. %, PA1 manganese (Mn): from 0.05 to 0.3 wt. %, PA1 phosphorus (P): up to 0.05 wt. %, PA1 sulfur (S): up to 0.03 wt. %, PA1 soluble aluminum (sol.Al): from 0.01 to 0.08 wt. %, PA1 nitrogen (N): up to 0.004 wt. %, PA1 niobium (Nb): from 0.005 to 0.03 wt. %, PA1 titanium (Ti): from 0.005 to 0.03 wt. %, PA1 boron (B): up to 0.003 wt. %, and PA1 the balance being iron (Fe) and incidental impurities, PA1 carbon (C): up to 0.01 wt. %, PA1 nitrogen (N): up to 0.01 wt. %, PA1 titanium (Ti): up to 0.2 wt. %, PA1 niobium (Nb): up to 0.2 wt. %, PA1 where, PA1 and PA1 the balance being iron (Fe) and incidental impurities, PA1 carbon (C): up to 0.0050 wt. %, PA1 silicon (Si): up to 1.0 wt. %, PA1 manganese (Mn): up to 1.0 wt. %, PA1 titanium (Ti): from [48/14N(%)+48/32S(%)] to [3.times.48/12C(%)+48/14N(%)+48/32S(%)] wt. %, PA1 niobium (Nb): from [0.2.times.93/12C(%)] to [93/12C(%)] wt. %, PA1 aluminum (Al): from 0.005 to 0.10 wt. %, PA1 phosphorus (P): up to 0.15 wt. %, PA1 nitrogen (N): up to 0.0050 wt. %, PA1 sulfur (S): up to 0.015 wt. %, PA1 and PA1 the balance being iron (Fe) and incidental impurities; PA1 carbon (C): under 0.0030 wt. %, more preferably, from 0.0010 to 0.0015 wt. %, PA1 silicon (Si): up to 0.05 wt. %, PA1 manganese (Mn): from 0.05 to 0.20 wt. %, PA1 phosphorus (P): up to 0.02 wt. %, PA1 sulfur (S): up to 0.015 wt. %, more preferably, up to 0.010 wt. %, PA1 acid-soluble aluminum (sol.Al): from 0.025 to 0.06 wt. %, PA1 nitrogen (N): up to 0.0030 wt. %, PA1 titanium (Ti): from 0.02 to 0.10 wt. %, more preferably, from 0.02 to under 0.07 wt. %, PA1 boron (B): from 0.0003 to 0.0010 wt. %, and the balance being iron (Fe) and incidental impurities, PA1 carbon (C): under 0.0030 wt. %, more preferably, from 0.0010 wt. % to 0.0015 wt. %, PA1 silicon (Si): up to 0.05 wt. %, PA1 manganese (Mn): from 0.05 to 0.20 wt. %, PA1 phosphorus (P): up to 0.02 wt. %, PA1 sulfur (S): up to 0.015 wt. %, more preferably, up to 0.010 wt. %, PA1 acid-soluble aluminum (sol.Al): from 0.025 to 0.06 wt. %, PA1 nitrogen (N): up to 0.0030 wt. %, PA1 titanium (Ti): from 0.02 to 0.10 wt. %, more preferably, from 0.02 to under 0.07 wt. %, PA1 boron (B): from 0.0003 to 0.0010 wt. %, and PA1 the balance being iron (Fe) and incidental impurities, PA1 n: number of roll stands of a finishing-rolling train in a hot-rolling mill, PA1 t.sub.0 : thickness of a steel sheet on the entry side of the first roll stand of said finishing-rolling train, PA1 t.sub.n-3 : thickness of the steel sheet on the exit side of the n-3-th roll stand of said finishing-rolling train, PA1 t.sub.n-2 : thickness of the steel sheet on the exit side of the n-2-th roll stand of said finishing-rolling train, PA1 t.sub.n-1 : thickness of the steel sheet on the exit side of the n-1-th roll stand of said finishing-rolling train, and PA1 t.sub.n : thickness of the steel sheet on the exit side of the n-th roll stand of said finishing-rolling train, PA1 where,
(1) 48/14[N(%)-0.002(%)]&lt;Ti(%), and PA2 (2) Ti(%)&lt;[4.00 C(%)+3.43N(%)], PA2 (1) (11/93)Nb-0.0004.ltoreq.B.ltoreq.(11/93)Nb+0.004, PA2 (2) Ti&gt;(48/12)C+(48/14)N, PA2 (3) Nb&lt;1/5.multidot.(93/48)Ti, and PA2 (4) C+(12/14)N+(12/11)B&gt;0.0038, PA2 (C/12+N/14)&lt;(Ti/48+Nb/93) PA2 where, a value of index (X) representing a content rate of titanium to boron, as calculated by the following formulae (1) and (2), is within a range of from 9.2 to 11.2: EQU X=-ln{(C/Ti*)B} (1) PA2 in said formula (1): EQU Ti*=Ti-(48/14)N-(48/32)S&gt;0 (2). PA2 where, a value of index (X) representing a content ratio of titanium to boron, as calculated by the following formulae (1) and (2), is within a range of from 9.2 to 11.2: EQU X=-ln {(C/Ti*)B} (1) PA2 in said formula (1): EQU Ti*=Ti-(48/14)N-(48/32)S&gt;0 (2); PA2 X: said index calculated by said formulae (1) and (2); then
and
cold-rolling the resultant hot-rolled steel sheet; and then
continuously annealing the resultant cold-rolled steel sheet within a temperature region of from 700.degree. C. to Ac.sub.3 transformation point (hereinafter referred to as the "prior art 1").
The fundamental technical idea of the prior art 1 is to completely fix nitrogen and carbon in steel within steel, before the hot-finishing-rolling of a steel sheet, by converting nitrogen in steel into titanium nitride (TiN) and converting carbon in steel into niobium-titanium carbides ([Nb-Ti]C).
(2) As described above, addition of boron in a slight amount to IF steel is very effective in inhibiting secondary-work embrittlement of a cold-rolled steel sheet, while causing deterioration of deep drawability of the cold-rolled steel sheet. Therefore, addition of boron to IF steel has not conventionally been considered the best practice. As a method for manufacturing a cold-rolled steel sheet having desired properties, which uses Nb-Ti-IF steel positively added with boron, as a material, Japanese Patent Provisional Publication No. 63-317,625 published on Dec. 26, 1988 discloses a method for manufacturing an ultra-low-carbon cold-rolled steel sheet excellent in fatigue resistance at a spot-welding zone, which comprises the steps of:
hot-rolling a steel slab consisting essentially of:
at a finishing temperature within a range of from 700.degree. to 900.degree. C. and a coiling temperature within a range of from 300.degree. to 600.degree. C.; then
cold-rolling the resultant hot-rolled steel sheet at a reduction rate within a range of from 60 to 85%; then
continuously annealing the resultant cold-rolled steel sheet at a temperature within a range of from a recrystallization temperature to 780.degree. C.; and then
temper-rolling same at a reduction rate within a range of from [thickness (mm)+0.1] to 3.0% (hereinafter referred to as the "prior art 2").
The fundamental technical idea of the prior art 2 is to ensure a sufficient strength of a welding heat-affected zone and a satisfactory deep drawability of a cold-rolled steel sheet by refining the structure of the welding heat-affected zone through addition of boron together with titanium and niobium to steel to prevent deterioration of strength of the welding heat-affected zone, which is an inevitable defect of IF steel.
(3) As a method for manufacturing a cold-rolled steel sheet excellent not only in resistance to secondary-work embrittlement, but also in surface treatability such as uniformity and glossiness of a plating layer, which uses Nb-Ti-IF steel added with boron, as a material, Japanese Patent Provisional Publication No. 59-140,333 published on Aug. 11, 1984 discloses a method for manufacturing a cold-rolled steel sheet for deep drawing excellent in resistance to secondary-work embrittlement and surface treatability, which comprises the steps of:
hot-rolling a steel slab consisting essentially of:
at a starting temperature of at least 950.degree. C.; then
cold-rolling the resultant hot-rolled steel sheet; and then
recrystallization-annealing the resultant cold-rolled steel sheet (hereinafter referred to as the "prior art 3").
The fundamental technical idea of the prior art 3 is to add boron to improve resistance to secondary-work embrittlement, and limiting the amount of added boron to a slight amount to improve surface treatability.
(4) As a method for manufacturing an alloying-treated zinc dip-plated cold-rolled steel sheet having an improved resistance to secondary-work embrittlement and a deep drawability kept constant, which uses Ti-IF steel added with boron, as a material, Japanese Patent Provisional Publication No. 1-184,227 published on Jul. 21, 1989 discloses a method for manufacturing an alloying-treated zinc dip-plated cold-rolled steel sheet excellent in deep drawability, which comprises the steps of:
hot-rolling a steel slab consisting essentially of:
at a final reduction rate of up to 20% in a finishing-rolling; then
cold-rolling the resultant hot-rolled steel sheet; then
subjecting the resultant cold-rolled steel sheet to a continuous zinc dip-plating treatment; and then
subjecting the thus formed zinc dip-plating layer to an alloying treatment (hereinafter referred to as the "prior art 4").
The fundamental technical idea of the prior art 4 is to improve deep drawability of an alloying-treated zinc dip-plated cold-rolled steel sheet by specifying a hot-rolling condition of a cold-rolled steel sheet.
(5) In a method for manufacturing a cold-rolled steel sheet including the hot-direct rolling method comprising directly hot-rolling a high-temperature continuously cast slab without reheating same, it has been difficult to manufacture a cold-rolled steel sheet for deep drawing having an excellent non-aging property on a similar level to that available in a method for manufacturing a cold-rolled steel sheet including the usual hot-rolling method comprising once cooling a high-temperature continuously cast slab, then reheating same, and then hot-rolling same. As a method for manufacturing a cold-rolled steel sheet excellent in non-aging property and deep drawability, based on the hot-direct rolling method, which solves this problem, Japanese Patent Provisional Publication No. 62-278,232 published on Dec. 3, 1987 discloses a method for manufacturing a cold-rolled steel sheet for deep drawing excellent in non-aging property, based on the hot-direct rolling method, which comprises the steps of:
directly hot-rolling a high-temperature continuously cast steel slab consisting essentially of:
without preheating same, with the use of a hot-rolling mill which comprises a roughing-rolling train and a finishing-rolling train;
limiting, when carrying out said hot-rolling, a reduction rate at two roll stands on the exit side of said roughing-rolling train to at least 45%, respectively, limiting an accumulative reduction rate at said two roll stands on the exit side of said roughing-rolling train to at least 70%, limiting an accumulative reduction rate at two roll stands on the entry side of said finishing-rolling train to at least 70%, limiting an accumulative reduction rate at two roll stands on the exit side of said finishing-rolling train to up to 20%, and completing said hot-rolling at a finishing temperature of at least 880.degree. C.;
coiling the resultant hot-rolled steel strip at a temperature within a range of from 640.degree. to 800.degree. C.;
cold-rolling said hot-rolled steel strip at a reduction rate within a range of from 70 to 90%; and
continuously annealing the resultant cold-rolled steel strip within a temperature region of from a recrystallization temperature to an Ac.sub.3 transformation point (hereinafter referred to as the "prior art 5").
The fundamental technical idea of the prior art 5 is to limit accumulative reduction rates in the roughing-rolling train and the finishing-rolling train of the hot-rolling mill, based on the hot-direct rolling method, thereby improving non-aging property and deep drawability of a cold-rolled steel sheet.
(6) It is known that a cold-rolled steel sheet for ultra-deep drawing is available by cold-rolling a hot-rolled steel sheet at a high reduction rate of from about 75% to about 90%. It is however practically difficult to adopt such a high cold-rolling reduction rate because of the construction and the capacity of a cold-rolling mill. As a method for manufacturing a cold-rolled steel sheet for ultra-deep drawing, which solves the above-mentioned problems, Japanese Patent Provisional Publication No. 1-294,823 published on Nov. 28, 1989 discloses a method for manufacturing a cold-rolled steel sheet excellent in ultra-deep drawability, which comprises the steps of:
hot-roughing-rolling a steel slab consisting essentially of:
at a temperature within a range of from 900.degree. to 1,200.degree. C., to precipitate carbide and nitride of titanium and/or niobium, thereby reducing the total content of solid-solution carbon and solid-solution nitrogen to up to 20 ppm;
hot-finishing-rolling the thus roughing-rolled steel slab at a temperature within a range of from 880.degree. to 660.degree. C., with the use of rolling rolls of which the ratio of a roll diameter (D.sub.1) to a finished thickness (t.sub.1) satisfies the following formula: EQU D.sub.1 &gt;100t.sub.1
at a reduction rate (R.sub.1) within a non-recrystallization temperature region;
coiling the resultant hot-rolled steel strip at a temperature of up to 600.degree. C.;
cold-rolling said hot-rolled steel strip, with the use of rolling rolls of which the ratio of a roll diameter (D.sub.2) to a finished thickness (t.sub.2) satisfies the following formula: EQU D.sub.2 &gt;100t.sub.2
at a reduction rate (R.sub.2) satisfying the following formula: EQU R.sub.2 &gt;50%
where, 95%&gt;(R.sub.1 +R.sub.2)&gt;75%; and
annealing the resultant cold-rolled steel strip (hereinafter referred to as the "prior art 6").
The fundamental technical idea of the prior art 6 is to improve a crystal texture of a cold-rolled steel sheet by limiting the ratio of the roll diameter of the rolling rolls to the finished thickness of the steel sheet in the hot-rolling and the cold-rolling, thereby improving deep drawability of the cold-rolled steel sheet.
(7) As a method for manufacturing a cold-rolled steel sheet excellent in deep drawability, in which a further higher synergistic effect brought about by the coexistence of niobium and titanium in Nb-Ti-IF steel is remarkably exhibited, Japanese Patent Provisional Publication No. 61-276,927 published on Dec. 6, 1986 discloses a method for manufacturing a cold-rolled steel sheet excellent in deep drawability, which comprises the steps of:
hot-finishing-rolling a steel slab consisting essentially of:
starting a cooling of the resultant hot-rolled steel strip within two seconds from the completion of said hot-finishing-rolling of said steel slab, cooling said hot-rolled steel strip at an average cooling rate of at least 10.degree. C./second before a start of coiling of said hot-rolled steel strip, and coiling said steel strip at a temperature of up to 710.degree. C.;
cold-rolling said hot-rolled steel strip at a reduction rate of at least 50%;
subjecting the resultant cold-rolled steel strip to a continuous annealing treatment which comprises heating said cold-rolled steel strip at a heating rate of at least 5.degree. C./second to a temperature region of from 400.degree. to 600.degree. C., and then, soaking same at a temperature within a range of from 700.degree. C. to an Ac.sub.3 transformation point for more than a second (hereinafter referred to as the "prior art 7").
The fundamental technical idea of the prior art 7 is to improve deep drawability of a cold-rolled steel sheet by limiting the timing of the start and the end of cooling of a hot-rolled steel strip during a period from the completion of hot-finishing-rolling to the start of coiling.
Along with the recent tendency toward more and more complicated and larger automobile parts and placing importance on rust preventiveness thereof, there is increasing the scope of application of a cold-rolled steel sheet for ultra-deep drawing of the EDDQ-class, which has so far been used only for portions requiring a severe press-forming (for example, a rear quarter portion), and EDDQ-class cold-rolled steel sheets are now being used in large quantities.
For the purpose of improving productivity of cold-rolled steel sheets, on the other hand, a continuous annealing of a cold-rolled steel sheet has become more popular. The continuous annealing, being carried out at a relatively high cooling rate, is suitable for annealing an ultra-low-carbon cold-rolled steel sheet. Under such circumstances, cold-rolled steel sheets made of IF steel which is ultra-low-carbon steel, have now been manufactured in large quantities through the continuous annealing. As described above, however, Ti-IF steel has an inevitable problem of secondary-work embrittlement. A careful consideration should therefore be taken when determining a chemical composition of Ti-IF steel.
In the prior arts 1 and 2, however, it is necessary to limit the niobium content in steel within a very tight appropriate range. In the prior arts 3 and 4, no regard is given to the improvement of balance between deep drawability and resistance to secondary-work embrittlement. In the prior arts 5 to 7, the appropriate relationship between the boron content in steel and the distribution of reduction rates during the hot-finishing-rolling, is not considered at all. When mass-producing cold-rolled steel sheets made of IF steel as a general-purpose breed, therefore, the problems intrinsic to IF steel such as secondary-work embrittlement may become more apparent. Sufficient care should therefore be taken upon determining a chemical composition of the cold-rolled steel sheet.
An object of the present invention is therefore to provide a chemical composition of a cold-rolled steel sheet, which is the most suitable for achieving a good balance between deep drawability and resistance to secondary-work embrittlement, which are two contradictory properties of a cold-rolled steel sheet made of IF steel, by solving the above-mentioned problems, and further to provide a method for manufacturing a continuously annealed cold-rolled steel sheet excellent in balance between deep drawability and resistance to secondary-work embrittlement, having the most desirable chemical composition as described above.